Date of Award

12-1-2011

Degree Name

Doctor of Philosophy

Department

Engineering Science

First Advisor

Mohanty, Manoj

Abstract

Coal spirals are widely used in coal preparation plants around the world to clean fine coal, typically in the 1 x 0.15 mm size range. Despite their popularity and the trend toward increased automation in modern coal preparation plants, adjustments to the critical process variable for coal spirals, i.e., product splitter position, continue to be done manually. Since spiral feed in a plant tends to fluctuate on a regular basis, timely manual adjustment of splitter position in tens or hundreds of spirals operating in a plant is nearly impossible. As a result, the clean coal yield from a spiral and also the overall plant suffers on a regular basis. The main goal of this study was to develop a suitable sensor and control system to adjust the product splitter position of a full-scale spiral. Some of the basic properties of coal slurry were thoroughly investigated for their on-line measurability and for their correlations with the density of the constituent solid particles. After experimenting with electrical capacitance- and conductivity- (i.e., reciprocal of resistivity) based sensing techniques, a conductivity-based tube sensor was developed for measuring density of solid particles in the spiral trough. Two sensors were used to establish a density gradient in the critical region across the spiral trough at the discharge end. Based on this continuously monitored density gradient, a PIC24 microcontroller was programmed to send a signal to a DC gear motor that would move the splitter arm in the appropriate direction when sufficient variation in conductivity was detected. Currently, a cycle time of 5 minutes is used for the spiral control system; however, in a commercial application, the cycle time could be lengthened to 30 or 60 minutes. The automation system has been validated by examining the performance of a full-scale spiral while deliberately changing factors like feed solid content, feed washability characteristics, and feed slurry ionic concentration. With a full-scale compound spiral programmed to achieve a specific gravity of separation at 1.65 by an automatic adjustment of the splitter position, the actual D50 values achieved for two separate tests were 1.64 and 1.73. The close proximity of target and actual D50 values is indicative of the effectiveness of the developed system. The next step in near-term commercialization of this proprietary spiral control system will be a longer term (several months) in-plant demonstration. The main goal of this study was to develop a suitable sensor and control system to adjust the product splitter position of a full-scale spiral. One of the basic properties of coal slurry was thoroughly investigated for its on-line measurability and for its correlation with the constituent solid density of the slurry. After experimenting with electrical capacitance- and conductivity- (i.e., reciprocal of resistivity) based sensing techniques, a conductive-based tube sensor was selected for measuring solids density of particles in the spiral trough. Two sensors were used to establish a density gradient in the critical region across the spiral trough at the discharge end. Based on this continuously monitored density gradient, a PIC24 microcontroller was programmed to send a signal to a DC gear motor that would move the splitter arm when sufficient variation in conductivity was detected. Currently, a cycle time of 5 minutes is used for the spiral control system; however, in a commercial application, the cycle time could be lengthened to 30 or 60 minutes. The automation system has been validated by examining the performance of a full-scale spiral while deliberately changing factors like feed solid content, feed washability characteristics, and feed slurry ionic concentration. With compound spirals programmed to achieve a specific gravity of separation at 1.65, actual D50 values achieved for two separate tests were 1.64 and 1.73. The close proximity of target and actual D50 values is indicative of the effectiveness of the developed system. The next step in near-term commercialization of this proprietary spiral control system will be a longer term (several months) in-plant demonstration.

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